static SinglyLinkedListNode mergeLists(SinglyLinkedListNode head1, SinglyLinkedListNode head2) {
        
        
        SinglyLinkedListNode head = new SinglyLinkedListNode(0); 
        SinglyLinkedListNode temp = head; //temp head, use for traversal
        SinglyLinkedListNode node1 = head1;
        SinglyLinkedListNode node2 = head2;
        while(node1 != null && node2 != null){
            if(node1.data < node2.data){
                temp.next = node1;
                node1 = node1.next;
            }else{
                temp.next = node2;
                node2 = node2.next;
            }
            temp = temp.next;
        }
        
        if(node1 != null){
            temp.next = node1;
        }
        
        if(node2 != null){
            temp.next = node2;
        }
        
        return head.next;
    }